Xmodem protocol!
once the protocol is put ino effect, the transmitter waits for the reciever to s
end a NAK (ASCII 21) Meanwhile, the reciever sends NAK every 10 seconds. Once th
e transmitter detects NAK, it starts to send the file in 128 byte blox.
actually more than 128 characters are sent- a typical data block looks like this

1- bye SOH (ascii 1)
1- byte block #
1- byte compliment of the block # (255-#)
1 block of 128 data bytes
1-byte checksum, discarding any carry
the way this shit works is like this-
(1) Transmitter waits for reciever to send NAK
(2) Transmitter sends block
(3) Reciever checks block out
(4) If block is good, send ACK (6) if not send NAK
at the end of the file,transmitter sends EOT (4) the reciever acknowledges with 
ACK, and it ends
Keep in mind that XMODEM must transmit & recieve 8 BIT WORDS